40. D 推理判断题。根据第二段最后一句中的"the main reason is that they share the same habits of eating too much and exercising too little”可知，作者认为发胖跟不均衡的生活方式有关。故选D项。

41. A 细节理解题。根据第四段第三句"The key is to develop a balanced and varied program that's fun as well as progressive"可知，进行多样化的锻炼会更有意思。故选A项。

42. B 细节理解题。根据最后一段第三句"Indeed, pain is your body telling you something's wrong and continuing to exercise could lead to serious injury.’’可知，如果感觉到持续的疼痛就要停止锻炼，否则会导致严重的伤害。故选B顶。